minor #6420 Added tip for optional second parameter for form submissions. (Michael Phillips)

This PR was merged into the 2.3 branch.

Discussion
----------

Added tip for optional second parameter for form submissions.

| Q              | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | all
| Fixed tickets | #6415

Commits
-------

153219a Added tip for optional second parameter for form submissions.

Author: xabbuh

cookbook/form/direct_submit.rst

@@ -84,6 +84,13 @@ method, pass the submitted data directly to
 
         $form->get('firstName')->submit('Fabien');
 
+.. tip::
+
+    When submitting a form via a "PATCH" request, you may want to update only a few
+    submitted fields. To achieve this, you may pass an optional second boolean
+    parameter to ``submit()``. Passing ``false`` will remove any missing fields
+    within the form object. Otherwise, the mising fields will be set to ``null``.
+
 .. _cookbook-form-submit-request:
 
 Passing a Request to Form::submit() (Deprecated)